1. Field of the Invention
This invention relates to ink jet printers and more particularly, to a method and apparatus for compensating for a voltage drop of pulse signals applied to heater elements of an ink jet printhead to enhance the quality of printing.
2. Description of Related Art
A thermal ink jet printhead selectively ejects droplets of ink from a plurality of drop ejectors to create a desired image on a copy surface. The printhead typically comprises an array of drop ejectors that convey ink to the copy surface. The printhead may move back and forth relative to the copy surface to print the image. Alternatively, the array may extend across the entire width of the copy surface. In either case, the copy surface moves perpendicularly relative to the linear array of the printhead. The ink drop ejectors typically comprise ink passageways, such as capillary channels, having nozzle ends and are connected to one or more ink supply manifolds. Each channel typically has a heater element for heating the ink. Ink from the manifold is retained within each channel until, in response to an appropriate signal, the ink in the channel is rapidly heated and vaporized by the heater element disposed within the channel. This rapid vaporization of some of the ink creates a bubble that causes a quantity of ink or droplet to be ejected through the nozzle to the copy surface. U.S. Pat. No. 4,774,530 to Hawkins shows the general configuration of a typical ink jet printhead. In order to enable high speed printing it is necessary to have multiple jets that are able to print simultaneously, as required by the pattern to be printed. For example, in a typical commercially available 128 jet printhead, up to 4 jets are fired at a time.
In conventional ink printing devices, the voltages applied across the heater elements are typically 10% over the "threshold" potential (the lowest voltage at which drops are ejected). However, if the voltage is set too high, the printhead degrades earlier due to kogation (ink residue) and results in heater failure.
Several prior art devices have attempted to control the temperature of the printhead to control the droplet and subsequent spot size.
For example, U.S. Pat. No. 4,980,702 to Kneezel et al. discloses a temperature control system that utilizes a control circuit that regulates heater operation to maintain the printhead in a desired operating range.
However, controlling the temperature of the printhead is difficult because to achieve a constant temperature range requires a large feedback time to sense the temperature, regulate the heater and check the regulated temperature.
To overcome the difficulties of directly controlling the temperature of the printhead, U.S. Pat. No. 5,223,853 to Wysocki et al., proposes selectively applying an electrical input signal having an amplitude and time duration to the heater elements to control the size of the ejected ink droplet.
It is further known that the size of a discharged droplet is determined by various controlling factors such as electrical energy quantity, as discussed in U.S. Pat. No. 4,345,262 to Shirato et al.